1. Field of the Invention
This invention relates to a by-pass and pressure unloader valve for use with a positive displacement pump system. It further relates to a valve which serves the functions of regulating pressure when the output is open and shifting to an unloading mode in which inlet presure is low to prevent excessive pressure when output is closed.
2. Description of the Prior Art
An example of a prior art valve for performing as a pressure control and unloader is the "Valve Construction" taught in U.S. Pat. No. 3,157,198, issued Nov. 17, 1964. This prior art valve included a structure with four distinct chambers, two check valves, a diaphragm and a biasing spring, resulting in a complex and inefficient structure.
More recently, improvements have been made on unloader valves. Specifically, the "Pressure Control and Unloader Valve" described in U.S. Pat. No. 3,953,154, issued Apr. 27, 1976, has fewer chambers and uses no distinct diaphragm, resulting in a simpler structure. It also, however, uses a valve assembly including a separate spring-loaded valve assembled from numerous parts at its outlet. This assembly requires a pressure wave to shift into the unloading mode. Finally, a single seal around the valve assembly absorbs all the wear due to movement, including sudden movement due to pressure waves.
Another development in the art is represented by the "High Pressure Unloader Valve" described in U.S. Pat. No. 4,292,990, issued Oct. 6, 1981. That device has a hollow cylindrical tube connecting the inlet and outlet openings. Around the tube is an opening into a by-pass, together with a series of parts which open a valve between the inlet and by-pass openings. This construction similarly uses a large number of moving parts in a relatively complex arrangement. Several seals are used which may be subject to wear.
Therefore, although the prior art teaches valves which perform the functions of regulating pressure and also of relieving sudden high pressure with a by-pass or unloader valve, the prior art valves are relatively complicated, and may be short lived due to seal wear.